Cavity resonator mode suppression means



April 15, 1952 H. B. BREHM 2,593,095

cAvITY RESONATOR MODE SUPPRESSION MEANS Filed June 29. 194e 2 SHEETS-SHEET 1 FIG?.

/NVENTOR H. B. BRE HM By uw A T TORNEV April 15, 1952 H. B.' BREHM OAVITY RESONATOR MODE SUPPRESSION MEANS 2 SHEETS-SHEET 2 Filed .June 29, 194e /Nl/ENYUR HBBREHM ATTORNV Patented Apr. 15, 1952 PATENT OFFICE CAVITY RESONATO M v M ODE SUPPRESSION ANSV Harold B. Brehm, Lyndhurst, N. J., assigner to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application June 29, 1946, Serial No. 680,367

6 Claims. (Cl. 1753-44) This invention relates to cavity resonators and more particularly to suppression of unwanted modes of oscillation in such resonators.

Objects of the invention are to improve the Q or selectivity characteristic of the resonatolg'to 5 enable discrimination between two sets of oscillations of substantially the same frequency but of different modes, and to renderI more stable the performance of a, cavity resonator having a variable position end wall or variable tuner so 1o designed as to provide the desired selectivity and discrimination.

Two generally distinct classes of oscillations which may occur in cavity resonators of cylindrical type are known respectively as transv ,15

verse magnetic, TM, and transverse electric Tlil." The transverse magnetic class is characterized by magnetic elds which extend in planes perpendicular or transverse to the cylindrical axis, the transverse electric class 'by electric fields which20 exist only in planes perpendicular to vthe cy-`I lindrical axis.

In accordance with one embodiment of the cylindrical chamber and of the movable end wall may all be of highlylelectrically conductive 3() material. The tuning piston of the resonator may fbe constructed as a sandwich consisting of `a plate of dielectric material between two me-y tallic coatings. or thin sheets. In order to discriminate between the longitudinal and radial 35 -electric vectors of the TM oscillations on the4 one hand and the transverse and circumferential electric vectors of TE modes of oscillation, on the other hand, longitudinal slots may be out entirely through the cylindrical wall of the 40 resonator for a portion of its length and similar rad1al slots may be provided through the end plates of the resonator. This brings about a relatively high discrimination in favor of the oscillations of TM mode and against the oscilla- 45 tions of the TE mode, the electric vector paths of which are intercepted by the slots in the cylindrical walls and end walls of the resonant chamber.

In the drawing, Fig. l presents in a kvertical 50 section the structure of a cylindrical cavity reso- 4 nator constituting one embodiment of the invention;

Fig. 2 is a horizontal section of the structure ofvFig. 1 along the plane of broken line 2-2 55 viewed inthe direction of the arrows;

Fig. 3 is a vertical section of a cylindrical cavity resonator similar to that of Fig. 1 but which is Operated in a different transverse magnetic mode of oscillation;

Fig. 4 is a horizontal section of the structure of Fig. 3 taken at the plane indicated by the dash lines 4-4 looking in the direction of the arrows;

Fig. 5 presents a modication of the structure of Fig. 3; and

Fig. 6 is a section along the horizontal plane through line 6-6 of Fig. 5.

Referring to Fig. 1, the cylindrical structure I0 having at its open upper end a closure or cover II may consist of any suitable rigid structural material provided that the interior surfaces vof the resonant chamber are highly' electrically conductive. A manual tuning control mechanism I2 on the cover II serves through a connection from the crank arm I3 to the pivotally mounted crosshead I4, the flat spring I5 and a pistonrod I6 to vary the position of a tuning piston I'I'supported by the pistonv rod IB and fitting -closely within the cylindrical portion I0 of the resonator. The lower surface of the piston face is provided with a peripheral annular plate I8 of spring metal slotted transversely along one margin and attached in any desired'manner to the piston. The slotted margin is ared outwardly and presents a closely spaced array of flat curved contact fingers IB which engage the cylindrical wall in sliding contact relation to maintain good electrical contact between the pistoniand the cylindrical walls. The spring fingers I9 substantially close the peripheral gap about the piston I1 from an electromagnetic standpoint so vfar as the desired TM mode oscillations are concerned. However, the slits between the ngers Apresent a certain dissipative leak for the fields of transverse electric modes of oscillation.

Input and output coaxial circuits` 20 and 2| respectively terminate in coupling loops 22 and 23 which pass through apertures 24 and 25 in the cylindrical wall of the resonator I0 and are electrically connected to the inner wall.

When Oscillation energy ofa natural frequency of a TM resonance of the cavity resonator is supplied to the internal eld space of the resonance chamber by coupling loop 22 the result'- ing TM mode eld presents electric vectors which extend longitudinally of the resonator between the nodal planes parallel to the end plates and which extend radially in the region of the nodal planes. -The eld of any oscillations of transverse electric mode oscillations which there is a tendency to excite within the resonator presents electric. vectors which lie wholly within planes transverse to the longitudinal axis of the resonator and have a configuration such as to be intercepted by radii in these respective planes. Advantage is taken of these factsto impose high attenuation for oscillations of the unwanted transverse electric mode by cutting entirely through the wall of the resonator equidistant slots parallel to the longitudinal axis of the resonator and extending for a considerable portion of the length of the resonator. Two of these slots 21 and 28 appear in Fig. l and it will be noted that their planes are substantially aligned respectively with the planes of coupling loops 22 and 23. Since the slots 21 and 28 are parallel to the direction of the longitudinal electric vectors of the transverse magnetic mode oscillations they will impose little attenuation for these transverse magnetic oscillations. However. transverse electric vectors directed in general in loops planes perpendicular to the slots will tend to set up diiferences of potential across the :slots .thus giving rise to a leakage of the field with consequent attenuation of these transverse electric oscillations. The two opposing electrically conductive surfaces constituting the end boundaries of the resonator determine nodal planes vbetween which the transverse electric vectors lie.

The apparatus of Fig. 1 may serve as a variable frequency electric selector to receive microwaves from circuit N and to select those of its own natural resonance frequencies and to discriminate strongly against Vthose of undesired modes and frequencies. It may be used to analyzethespectrurn of a complex wave by .selecting in: tum, as its tuning piston moves, the components of the band of frequencies through which, the tuning mechanism enables Vit to respond. It may be especially useful as a phantom traget or echo box for a radar system in which outgoing pulses from the radar transmitter may be yimpressed upon the resonator through input circuit `20 and the `resulting electromagnetic field built up within the resonator may upon cessation-,of each transmitter pulse return a protracted train of microwaves to enable tuning and testing the radar receiver as well as checking up on the performance of the radar system as a whole'.

Fig. 2 illustrates an expedient for increasing the attenuation suffered by the transverse electric oscillations. As shown in that figure, the metallic end surface (iii of the piston Il which may consist of a thin copper valuminum or silver plate is cut through along radial lines as indicated at'3l, 32, 33 and 34. Consequently, electric vectors of Vtransverse electric oscillations in the region of the nodal plane tend to set up potential differences between the opposite margins of the slots 3l, 32, etc. with a resulting tendency for leakage of the transverse electric eld into the dielectric disc 35 which constitutes the main centralportion of the movable piston i1. The dielectric substance serves to substantially attenuate the transverse electric field. It also serves as the structural support for the electrically conductive surface 3i). Under conditions of heat and moisture there may be a tendency for the piston I1 to be warped. Any such effect which disarranges the nice parallelism which should obtain between the conductive end surfaces 30, 30' may seriously distort and reduce the desired oscillation viield within'the intervening space. In order to prevent such warping or distorting the piston Il is provided with a back metallic plate 3l which. tends both to stiifen the piston Il as a whole and prevent warping and also serves to preclude entrance of moisture into the piston structure with its consequent inimical effects. It will therefore be apparent that the sandwich construction of the piston Il permits advantage to be taken of the lower mass which a dielectric structure may possess, of the energy-dissipating characteristic of the dielectric for the transverse electric eld to which it is subjected through the slots 3| to 3d, and the increased rigidity which the composite structure possesses. l

If desired, the lower end 33 of the resonance chamber Vlil may be slotted radially in a manner entirely analogous to the lower conducting surface 3o of the piston Il.

The radial cuts in the conducting surface of the end plates have maximum effectiveness in suppression of such transverse electric oscillation modes as TE 1 1 n, TE 1 2 n, TE 1 a n when the radial cut is directly over a couplingr loop. For other positions of the slots the effectiveness for each mode family will diminish to a minimum when the coupling loops are positioned halfway between the adjacent slots.

The particular dielectric material to be used in the piston may depend upon such considerations as mass, freedom from moisture, and cost of manufacture. Among materials which may be used for this purpose are discs of wood and phenol bre. Conducting faces 30 utilize any highly electrically conductive material in the form of a thin plate or of an electrodeposition or coating. rEhe inner surface of the cylindrical wall of the resonator iii may be made highly electrically conductive in the same manner. it is preferable, however, that the metallic back plate 3l have sumcient thickness to impart the necessary rigidity to the piston structure as a whole in order .to preclude buckling or warping.

Figs. 3 and 4 disclose a modification of the structure of Figs. l and 2 in which the tuning piston except for the radial slots in the end surfaces and the speciiic form of coupler used may be identical with that of Fig. l. In this embodiment the integral lower end 49 of the cylinder 4| which serves as the lower electrical boundary of the resonant chamber has an upper surface made highly conductive by a plating or deposition of silver, copper or other good conducting material. As is indicated in Figs. 3 and 4 the end i0 is slotted entirely through in radial directions as .at 42, 13, E4, ll, l5 and 41. The lower conducting surface 36 of the piston 39 may be slotted in an analogous manner.

This structure presents little dissipation to modes of oscillation in which the electric vectors are entirely radial but acts to selectively suppress all modes of the TEU type and other T'l .modes which have electric vectors which must pass transversely to a radial slot of the end plate til or an aligned longitudinal slot 4B of the cyiindrical wall of cylinder 4i. The structure of Figs. 3 and 4 could support oscillations of 'IEa mode as, for example, 'IEa 3 12 and of TMs mode as, for example, TM3 312. Which of the two is selected largely may be determined by the nature and positioning of the coupling element. For example, with probe couplers such as 49 and 5i) positionally aligned with radial slots 46 and d3 and longitudinal slots dii and El and extending longitudinally into the i-leld the system may be excited in TMs mode of oscillations one of the couplers '49, 5i! serving as input and the other as output transducer. |The structure of Figs.

v 3 and 4 may be modied as in Figs. 5 and 6 to utilize magnetic loop couplers 53 and 54 positioned midway between radial slots and with their principal planes generally perpendicular to the slots and hence parallel to the electric vector of the TMs mode oscillations'. In the instance of the loop the couplers should be located at positions of substantial field intensity and hence should be separated electrically from the outer conducting Walls and from the internal nodal surfaces. In the instance of the probe couplers for TMs mode oscillations the preferable position is at a point one-third of the radius or twothirds of the radius from the center.

In Fig. the lower metallic surface or coating 5B may be slotted in the same manner as the lower end 51 of the resonator chamber.

The vertical slots in the cylindrical walls ein tend over the central portion of the length of these walls. YThis leaves the lower portions of the resonators free for introduction of couplers where desired.

What is claimed is:

1. A high Q cavity resonator comprising a cy" lindrical chamber having electrically conductive surfaces bounding the space therewithin, means for initiating natural resonance frequency oscillations of TM modes within the chamber, an end wall of said chamber having radial slits there through to attenuate and inhibit unwanted extraneous oscillations of TE modes, said slits having potential differences for TEu 1 n modes set up between opposite margins thereof.

2. A high Q cavity resonator comprising a cylindrical chamber having electrically conductive surfaces bounding the space therewithin, means for initiating natural resonance frequency oscillations of a TM mode within the chamber, an end wall of said chamber having slits extending along the central portions of radii of said walland having an oscillation energy transfer element extending through the opposite end Wall at a position aligned with one of said slits.

3. A high Q cavity resonator comprising a cylindrical chamber having electrically conductive surfaces bounding the space therewithin, an end Wall of said chamber being movably mounted with respect to the remainder of the chamber and one of the end walls of said chamber having slits therethrough extending in radial directions.

4. A cylindrical cavityresonator of electrically conducting material, means for initiating therewithin oscillations of a desired mode, one end of the resonator comprising a disc of extraneous mode absorbing dielectric material having a conducting surface facing the resonator and a stiiening plate physically connected to the disc of dielectric material on its surface remote from the resonator to prevent warping, slits in said surface converging toward a common center for suppressing TE modes.

5. A high Q cavity resonator comprising a chamber having conductive surfaces bounding the space therewithin, means for exciting natural resonance frequency oscillations Within said chamber, an end wall of said chamber being adjustable to tune said resonator, said end wall comprising a disc of attenuating dielectric material and two opposed conductive plates in hush contact with said dielectric material, and radial mode suppressing slits in one of said plates for producing leakage of extraneous modes into said dielectric.

6. A high Q cavity resonator comprising a cylindrical chamber having electrically conductive surfaces bounding the space therewithin, means for initiating natural resonance frequency oscillations of TM modes within the chamber, a wall of said chamber having longitudinal slits therethrough parallel to the electric vector of the TM Amode oscillations to attenuate and inhibit unwanted extraneous oscillations of TE modes, and a tuning piston movable in said resonator, said piston having TE mode suppression slits'radially disposed thereon, said radial slits being aligned with a plane containing said Wall slits, said radial slits leaving the TM modes substantially unattenuated.

HAROLD B. `BREHlVf.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,151,118 King Mar. 21, 1939 2,197,122 Bowen Apr. 16, 1940 2,233,263 Linder Feb. 2-5, 1941 2,267,289 Roosenstein 'Dec. 23, 1941 2,283,568 Ohl May 19, 1942 2,417,542 Carter Mar. 13, 1947 2,428,287 Linder Sept. 30, 1947 2,439,388 Hansen Apr. 13, 1948 2,449,855 Marholz Sept. 2l, 1948 2,471,419 Edson et al. May 31, 1949 2,500,417 Kinzer Mar. 14, 1950 

